There has been an explosion of interest in how features of the social and built environments of communities affect the physical activity of residents. Because of the relevance of walking as a component of physical activity, and the possibility of increasing population-levels of physical activity by increasing walking in daily life, the study of the environmental determinants of walking has received special attention. Studies have documented associations of walking with environmental features such as land use mix and proximity of destinations, street connectivity and presence of sidewalks, aesthetic and design features of the environment, and safety and violence levels. However, important questions remain regarding the policy implications of this work. A major challenge is the need to account for the multiple dynamic relations between individuals (e.g. the behavior of one individual affecting those of others around him or her), between individuals and their environments (e.g. the environment changing in response to the behaviors of individuals and vice versa), and between environments (e.g. street connectivity affecting levels of safety). Accounting for these dynamic relationships is fundamental to understanding how environmental factors operate and to identifying plausible effects of various policies. Systems science methodologies (including agent-based models or ABMs) have received increasing attention as a way to better capture the complex set of dynamic relationships inherent in population health problems, but few applications to specific research problems exist. The overall goal of this proposal is to develop a generic agent-based model (ABM) of people's walking behavior within a hypothetical city. This model will be used to enhance scientific understanding of the ways in which environmental factors affect population-levels of walking and socioeconomic inequalities in walking. The Specific Aims are: (1) To build a generic agent-based model to simulate people's walking behavior within a hypothetical city; (2) To use the proposed model to specifically investigate how mixed land use and safety could contribute to population- levels of walking and to socioeconomic inequalities in walking; and (3) To explore the feasibility (including data availability and new data needs) of making the model reflect a real city. This proposal will apply novel systems methodologies to a policy resistant population health problem and will generate one of the first exemplars of applications of this approach in population health. It will develop a generic model useful for understanding dynamics that operate across cities and will lay the ground work for further development of more realistic models specific to real cities. The systems approach we propose has great potential for improving our understanding of the environmental determinants of walking which will be crucial to identifying relevant policies and anticipating their effects in future research. The development of these models represents a paradigm change in the way evidence is used to select and support interventions and policies to prevent major chronic conditions, including cardiovascular disease and cancer.